Polymerization of propylene was carried out using a $MgCl_2$-supported $TiCl_4$ catalyst in conjunction with triethylaluminium (TEA) as the cocatalyst and various types of alkoxy silane compounds as an external donor. The effect of the external donor on the performance of the catalyst with different internal donors was investigated. The polydispersity index (PDI) of polypropylene (PP) obtained with the diether and succinate based catalyst were decreased with the introduction of an external donor and the PDI increased for the phthalate based catalyst. The molecular weight and PDI increased with the introduction of an external donor. The highest PDI of PP was obtained by polymerization with di-n-propyldimethoxysilane (DnPDMS) as an external donor. In addition, a mixture of external donors was used to control the PDI of PP and the composition of the catalyst was examined after treated with TEA/external donor. Furthermore, the theoretical PDI value was calculated for a mixture of external donor systems. The PDI of PP could be controlled and predicted while retaining high activity, high isospecificity and high molecular weight by changing the structure of the external donor and/or their mixture.